1. Software Requirements
Chapter 5
Software Requirements

2. Software Requirements
Descriptions and specifications of a system

3. Objectives To introduce the concepts of user and system requirements
To describe functional and non-functional requirements
To explain two techniques for describing system requirements
To explain how software requirements may be organized in a requirements document

4. What is a requirement?
Range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification
Requirements may serve a dual function
May be the basis for a bid for a contract - therefore must be open to interpretation
May be the basis for the contract itself - therefore must be defined in detail
Both these statements may be called requirements

5. Requirements engineering
The requirements are the descriptions of the system services and constraints
The requirements engineering is the process of establishing the customer services and addressing system constraints

6. Types of requirement User requirements System requirements
Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers
System requirements
A structured document setting out detailed descriptions of the system services. Written as a contract between client and contractor
Software specification
A detailed software description which can serve as a basis for a design or implementation. Written for developers

7. Requirements readers

8. Topics covered Functional and non-functional requirements
User requirements
System requirements
The software requirements document

9. Functional and non-functional requirements
Statements of services the system should provide, how the system should react and behave in a certain condition.
Non-functional requirements
Emergent properties such as reliability, response time, availability, not concerned with any specific functions delivered by the system.
Domain requirements
Requirements that come from the application domain of the system and that reflect characteristics of that domain

10. Functional requirements
Describe functionality or system services
Functional user requirements are high-level statements
Functional system requirements describe system services in detail

11. Examples of functional requirements
The user shall be able to search either all databases or specify a subset.
The system shall provide appropriate viewers for the user to read documents.
Every order shall be allocated a unique identifier (ORDER_ID).

12. Requirements imprecision
Problems arise when requirements are not precisely stated
Ambiguous requirements may be interpreted in different ways by developers and users
Consider the term ‘appropriate viewers’
User intention - special purpose viewer for each different document type
Developer interpretation - Provide a text viewer that shows the contents of the document

13. Requirements completeness and consistency
Typically, requirements should be both complete and consistent but may have difficulty in practice
Complete
The requirements should include descriptions of all facilities required
Consistent
There should be no conflicts or contradictions in the descriptions of the system facilities

14. Non-functional requirements
Define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.
Process requirements may also be specified mandating a particular CASE system, programming language or development method
Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless

15. Non-functional classifications
Product requirements
Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc.
Organizational requirements
Requirements which are a consequence of organizational policies and procedures e.g. process standards used, implementation requirements, etc.
External requirements
Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc.

16. Non-functional requirement types

17. Non-functional requirements examples
Product requirement
Performance requirements on how fast the system must execute
Organizational requirement
Process standards which must be used in the organization
External requirement
Legislative requirements which must be followed

18. Goals and requirements
Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify.
Goal
A general intention of the user such as ease of use
Verifiable non-functional requirement
A statement using some measure that can be objectively tested
Goals are helpful to developers as they convey the intentions of the system users

19. Examples A system goal A verifiable non-functional requirement
The system should be easy to use by experienced controllers and should be organized in such a way that user errors are minimised.
A verifiable non-functional requirement
Experienced controllers shall be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.

20. Requirements measures

21. Requirements interaction
Conflicts between different non-functional requirements are common in complex systems
Spacecraft system – an example
To minimise weight with few chips
To minimise power consumption with low power chips
However, using low power chips need more chips
Which is the most critical requirement?

22. Domain requirements
Requirements and features are for a specific domain.
Domain requirements have specific functions, constraints or computations
The system may not work if domain requirements are not satisfied

23. Train protection system
A specific computation function for train domain:
The deceleration of the train shall be computed as:
Dtrain = Dcontrol + Dgradient
where Dgradient is 9.81ms2 * compensated gradient/alpha and where the values of 9.81ms2 /alpha are known for different types of train.

24. Domain requirements problems
Understandability
Requirements are expressed in the language of the application domain, resulting in understanding problems for software engineers
Implicitness
Domain specialists understand the area so well that they do not think of making the domain requirements explicit

25. Topics covered Functional and non-functional requirements
User requirements
System requirements
The software requirements document

26. User requirements
Describe functional and non-functional requirements for users who don’t have technical knowledge
Use natural language, tables and diagrams

27. Problems with natural language
Ambiguity
Readers and writers may interpret the same words differently.
Requirements confusion
Functional and non-functional requirements tend to be mixed-up
Requirements amalgamation
Several different requirements may be expressed together
Over-flexibility
The same thing may be described in different ways in the specification
Lack of modularization
NL structures are inadequate to structure system requirements

28. Editor grid requirement
2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines.

29. Guidelines for user requirements
Invent a standard format and use it for all requirements
Use language in a consistent way.
Use text highlighting to identify key parts of the requirement
Avoid the use of computer jargon

30. Structured presentation
2.6 Grid facilities
2.6.1 The editor shall provide a grid facility where a matrix of horizontal and vertical lines provide a background to the editor window. This grid shall be a passive grid where the alignment of entities is the user's responsibility.
Rationale: A grid helps the user to create a tidy diagram with well-spaced entities. Although an active grid, where entities 'snap-to' grid lines can be useful, the positioning is imprecise. The user is the best person to decide where entities should be positioned.
Specification: ECLIPSE/WS/Tools/DE/FS Section 5.6

31. Topics covered Functional and non-functional requirements
User requirements
System requirements
The software requirements document

32. System requirements More detailed specifications of user requirements
Serve as a basis for designing the system
May be used as part of the system contract
System requirements may be expressed using system models (data-flow model, object model, etc.)

33. Requirements and design
Requirements care about “what the system should do”
Design cares about “how to fulfill the requirements”

34. Two Alternatives to NL specification
Structured natural language specifications
PDL-based definitions

35. Structured NL specifications
A limited form of natural language may be used to express requirements
This removes the problems of ambiguity and flexibility and supports uniformity

36. Structured NL -- Form-based

37. PDL-based requirements definition
A programming language like description but with more flexibility of expression
Appropriate in two situations
Where an operation is specified as a sequence of actions and the order is important
When interfaces have to be specified

38. Part of an ATM specification

39. PDL disadvantages
PDL may not be sufficiently expressive to express the system functionality in an understandable way
Notation is only understandable to people with programming language knowledge
The requirement may be taken as a design specification rather than a model to help understand the system

40. Topics covered Functional and non-functional requirements
User requirements
System requirements
The software requirements document

41. IEEE requirements standard
Introduction
General description
Specific requirements
Appendices
Index
This is a generic structure that must be instantiated for specific systems

42. Requirements document structure
Introduction
Glossary
User requirements definition
System architecture
System requirements specification
System models
System evolution
Appendices
Index

43. Requirements Specification – Volere Template
Table of Contents
Project Drivers
The Purpose of the Product
Client, Customer and Stakeholders
Users of the Product
Project Constraints
Mandated Constraints
Naming Conventions and Definitions
Relevant Facts and Assumptions
Functional Requirements
The Scope of the Work
The Scope of the Product
Functional and Data Requirements

44. Requirements Specification – Volere Template
Non-Functional Requirements
Look and Feel Requirements
Usability Requirements
Performance Requirements
Operational Requirements
Maintainability and Portability Requirements
Security Requirements
Cultural and Political Requirements
Legal Requirements
Project Constraints
Open Issues
Off-the-Shelf Solutions
New Problems
Tasks
Cutover
Risks
Costs
User Documentation and Training
Waiting Room
Ideas for Solutions

45. Key points
Requirements set out what the system should do and define constraints on its operation and implementation
Functional requirements set out services the system should provide
Non-functional requirements constrain the system being developed or the development process
User requirements are high-level statements of what the system should do

46. Key points
User requirements should be written in natural language, tables and diagrams
System requirements are intended to communicate the functions that the system should provide
System requirements may be written in structured natural language, a PDL or in a formal language
A software requirements document is an agreed statement of the system requirements